1. Field
The disclosure relates to a method, system, and article of manufacture for the concurrent enablement of persistent information unit pacing.
2. Background
Fibre Channel refers to an integrated set of architectural standards for data transfer being developed by the American National Standards Institute. Fibre Connection (FICON) is a protocol of the fibre channel architecture and may also be referred to by the formal name of FC-SB-3. Further details of FC-SB-3 may be found in the publication, “FIBRE CHANNEL Single-Byte Command Code Sets-3 Mapping Protocol (FC-SB-3)”, Rev. 1.6, published by the American National Standards for Information Technology on Mar. 26, 2003.
A channel is a direct or a switched point-to-point connection between communicating devices. In the Fibre Channel architecture, a FICON channel may perform the functions specified by FC-SB-3 to provide access to Input/Output (I/O) devices by means of control units or emulated control units. FICON channels may rely on packet switching for transferring data between communicating devices. In FC-SB-3, a channel may also be referred to as an entity, typically of a host computer, which includes one N_Port and elements which perform the functions specified by FC-SB-3 to provide access to I/O devices by means of control units or emulated control units. Also, in FC-SB-3, a control unit may comprise a physical or emulated entity that includes at least one N_Port and elements which adapt the characteristics of one or more I/O devices to allow the attachment of the I/O devices to the N_Port of a channel.
A channel command word (CCW) is a control block which includes an I/O request, and may refer to a structure of a specific system architecture which specifies a command to be executed along with parameters. A channel program is a sequence of one or more channel command words executed sequentially that controls a specific sequence of channel operations. FICON channels may in certain situations transmit up to sixteen channel command words at a time along with the associated data for any write operations, where a channel command word may be referred to as an “information unit” (IU).
The IU pacing protocol, as defined in the FICON architecture, has the limitation that the first burst of information units from the channel to the control unit may be no larger than a default value of 16. This may cause a delay in the execution of channel programs with more than 16 commands at large distances, such as distances of over a hundred kilometers (or at shorter distances as link speeds increase), because a round trip to the control unit may be needed before the remainder of the information units can be sent by the channel to the control unit. A control unit can adjust the value of the IU pacing parameter in the command response information unit, to modify the flow of information units within the FICON architecture, such that more than sixteen information units can be in flight at any point in time.
In FC-SB-3, a change in the IU pacing parameter remains in effect until the end of the current command chain or channel program. Certain published amendments to FC-SB-3 allow the pacing parameter to be persistent and remain in effect for any new command chains on the logical path. Such amendments to FC-SB-3 allow the FICON channel to retain a pacing count that can be used at the start of execution of a channel program. This may improve the performance of I/O programs at distances of over a hundred kilometers (or at shorter distances as link speeds increase) by allowing a channel to send the entire channel program to the control unit and by eliminating the delay of waiting for the first command response information unit. The channel retains the pacing count value, presented by the control unit in accordance with FC-SB-3, and uses that pacing count value as the new default pacing count for any new channel programs issued on the same logical path. Further details of allowing persistent IU pacing parameter in FC-SB-3 may be found in the publication “FIBRE CHANNEL Single-Byte Command Code Sets AMENDMENT1 (FC-SB-3/AM1)”, Rev. 1.02, published by the American National Standards for Information Technology on Feb. 7, 2007.
In Fibre Channel an extended link service (ELS) request solicits a destination port to perform a function. An ELS reply is transmitted in response to an ELS request, unless otherwise specified. A registered state change notification (RSCN) ELS may be sent to registered ports, in response to an occurrence of an event. A request node identification data (RNID) ELS is an ELS for acquiring node identification data. Extended link services for FICON may also be referred to as FC-LS and further details may be found in the publication “Fibre Channel Link Services (FC-LS)”, Rev. 1.62, published by the American National Standard for Information Technology on Dec. 4, 2006.